\begin{tikzpicture}
    \pgfmathsetmacro{\a}{2}
    \pgfmathsetmacro{\b}{1.5}

    \begin{scope}[yshift=2.6cm]
        \tkzDefPoints{0/0/b1, \b/0/b2, 0/.7/a1, \a/.7/a2}
        \tkzDrawSegments[xianduan={below=0pt}](b1,b2  a1,a2)
        \tkzLabelSegment[above](a1,a2){$a$}
        \tkzLabelSegment[above](b1,b2){$b$}
    \end{scope}

    % 1
    \tkzDefPoints{0/0/A, \a/0/P}
    \tkzDrawSegment(A,P)
    \tkzLabelSegment[below](A,P){$a$}
    \tkzLabelPoints[left](A)
    \tkzLabelPoints[below](P)

    % 2
    \tkzDefPoints{\a+\b/0/B}
    \tkzDrawSegment(P,B)
    \tkzLabelSegment[below](P,B){$b$}
    \tkzLabelPoints[right](B)

    % 3
    \tkzDefMidPoint(A,B)  \tkzGetPoint{O}
    \tkzDrawPoint(O)
    \tkzDrawArc(O,B)(A)

    % 4
    \tkzDefLine[perpendicular=through P](A,B)  \tkzGetPoint{c}
    \tkzInterLC(P,c)(O,B)  \tkzGetSecondPoint{C}
    \tkzDrawSegment(P,C)
    \tkzLabelSegment[left](P,C){$c$}
    \tkzMarkRightAngle(B,P,C)
    \tkzLabelPoints[above](C)
\end{tikzpicture}

